It is known to use both a focussed laser beam and a plasma arc to impinge at the same point on a metal workplace to be welded, but previously some arrangements have been limited to use of free-burning arcs such as the Tungsten-Inert-Gas (TIG) or Metal-Inert-Gas (MIG) type. For example, in UK Patents nos 1547172 and 1600796, Steen discloses the treatment of surfaces by use of a focussed laser beam and a free burning arc, but such an arrangement is difficult to control at start-up (when the arc may root on the nearest electrical ground point) and shut-down, so the results are insufficiently controllable for a commercially applicable process. Further, the use of a free-burning arc results in a relatively large area of workpiece heating compared to the area heated by the laser beam, so the benefits of a narrow heat affected zone are significantly reduced.
In a paper by Walduck and Biffen published in "Welding and Metal Fabrication", April 1994, the advantages of the use of a constricted plasma arc to augment laser welding are described.
CN-A-1065420 discloses an electron beam welding device in which the plasma arc is generated by a plasma gun installed at the lower end of a final-stage vacuum chamber.
SU-A-1 815 085 describes a laser arc processing apparatus for large thickness metal objects. The apparatus has a body, nozzle, an electrode and a lens positioned coaxially with the electrode. To obtain a plasma arc the electrode is hollow and water-cooled and is mounted with a gas vortex unit onto the water-cooled nozzle which is provided with an exit channel of the plasma arc. The lens is located within the electrode cavity and its focal point is positioned at the centre of the nozzle channel to focus the laser at a point in the arc plasma inside the nozzle of the plasma torch. The laser is not used for welding or cutting but to enhance the plasma arc performance without impinging on the workpiece surface. In addition, the body of the plasma torch acts as the electrode for arc formation.
In plasma arc augmented laser welding apparatus it is known to initiate the plasma arc by means of a pilot arc generated by applying a high voltage electrical signal between the electrode and the plasma nozzle but this method has the problem that it causes erosion of the electrode which can affect the plasma arc. Also the initiation of the plasma arc may not be reliable and consistent resulting in an unsatisfactory weld.